physics – Simulating a rocket out of control in Box2D

In the void of space, a spaceship releases a rectangle shaped space-torpedo. In half a second, it starts its rocket engine and goes straight until something get in its way, and then boom!
The code looks something like this on update():

if (Game::gameplay_timer_.milliseconds() - launch_time_ > thrust_time_) {
  body_->ApplyLinearImpulseToCenter(0.006f * body_->GetLinearVelocity(), true);

But, what if something disturbs the projectile before igniting the rocket, giving it some rotation? It should enter in an uncontrolled spiral (think KSP or tethered gas canisters in Just Cause 2).

How can I achieve this effect (spiraling uncontrollably)? I tried to apply the impulse in places other than the center using ApplyLinearImpulse(), but I can only make the projectile rotate on itself keeping the direction perfectly straight.